In [6]: list(itertools.chain(*[[1, 2], [3, 4]]))
Out[6]: [1, 2, 3, 4]

In [7]: list(itertools.chain.from_iterable([[1, 2], [3, 4]]))
Out[7]: [1, 2, 3, 4]

ну или в лоб:

In [8]: [x for xs in [[1, 2], [3, 4]] for x in xs]
Out[8]: [1, 2, 3, 4]